Method for producing ceramic objects

ABSTRACT

A process for forming a ceramic object using selective laser sintering of ceramic powder is provided. The chemical composition and size distribution of the powder may be varied between various regions of the object, and the temperature achieved by laser heating may be varied between regions to achieve the desired degree of densification. In one embodiment, a ceramic mold ( 1 ) is formed in to have an envelope portion ( 4 ) and a core portion ( 7 ) with differing properties using a fast prototyping laser sintering process.

CROSS REFERENCE TO RELATED APPLICATION

This application is the US National Stage of International ApplicationNo. PCT/US2004/010176, filed Apr. 01, 2004 and claims the benefitthereof. The International Application claims the benefits of U.S.Provisional Application No. 60/460,326, filed Apr. 4, 2003, both of theapplications are incorporated by reference herein in their entirety.

FIELD OF THE INVENTION

This invention relates to a process for producing ceramic objects, suchas ceramic components and ceramic molds, from a ceramic powder.

BACKGROUND OF THE INVENTION

For various purposes, e.g., test purposes, ceramic shaped objects oftenhave to be produced quickly. Hitherto, complex processes have been used,such as the pressing of ceramic powders with subsequent sintering andmachining to a final shape. Ceramic objects of this type may comprise,inter alia, mold shells which are used in casting. In this case too, itis desirable to produce mold shells quickly to allow rapid testing ofcastings, for example.

U.S. Pat. No. 6,446,697 has disclosed a process for producing complexcastings in which wax models are used in an intermediate step.

U.S. Pat. No. 6,273,326 has disclosed the production of ceramic shapedparts in which individual layers are joined to one another by means oflaser sintering.

U.S. Pat. No. 5,824,250 has disclosed the production of a ceramic shapedpart-by means of a gel that contains ceramic. This process requires theuse of a mold.

DE 198 53 834 A1 has disclosed a process for producing components byapplication technology. In this case, material has to be applied inlayers. This is very time-consuming.

DE 199 48 591 A1 or WO 01/26885 A1 discloses a rapid prototyping processusing a special application technology. The application technologyinvolves the material which is to be applied being present in a supportliquid.

U.S. Pat. No. 4,863,538, titled “Method and Apparatus for ProducingParts by Selective Sintering,” and U.S. Pat. No. 5,132,143, titled“Method for Producing Parts,” each discloses computer-aided laserapparatus for sequentially sintering a plurality of powder layers tobuild the desired part. Both of the foregoing patents are hereinincorporated by reference in their entirety.

It is known from a publication issued by the Institute for Ceramictechnologies and Sintered Materials entitled “Rapid Prototyping ofComplex-Shaped Parts of Si/SiC Ceramics by Laser Sintering” to use CADdata to sinter ceramic components by means of laser. However, theceramic components produced in this way are porous and have to beinfiltrated with liquid silicon.

The contribution “Prototyping of Complex-Shaped Parts and Tools ofSi/SiC Ceramics by Selective Laser Sintering” by Von W. Löschau, R.Lenk, Siegfried Scharek, M. Teichgräber, S. Nowotny, C. Richter, knownfrom the 9th Simtec-World Ceramics Congress: Ceramics: Getting into the2000s-Part B, likewise deals with the laser sintering of porous ceramiccomponents.

It is known from the prior art to produce porous ceramics and to fillthem with liquid silicon. This causes problems for high-temperatureapplications of a component of this type, since the silicon isvulnerable to re-melting during the use of the part.

SUMMARY OF THE INVENTION

Standard rapid prototyping processes can be adapted to produce improvedceramic shaped objects. Inter alia, the selective laser sintering ofceramic powders may be useful in this context. In one exemplaryembodiment, powder may be applied continuously or discontinuously, forexample in a powder bed, and may be densified (e.g., sintered) by heatproduced by laser energy directed at locations that correspond to thecomponent to be produced.

Aspects of the present invention provide a method for rapidly producinga ceramic object having different material properties at selectablelocations within the object. Laser energy may be used to sinter aceramic powder. Bt way of example, powder particle size, powder particlecomposition, and laser energy parameters may be selectively variedacross regions of the object. This may be useful for producing ceramicmold shells having a core portion and an envelope portion, wherein thedesired properties of the core and envelope are different.

BRIEF DESCRIPTION OF THE DRAWING

These and other advantages of the invention will be more apparent fromthe following description in view of the drawing that shows a ceramicmold shell formed by a process embodying aspects of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

According to aspects of the present invention, ceramic powders or powdermixtures with suitable grain sizes are used to produce a virtually denseshaped object using laser sintering, and further, to produce an objecthaving a material property that may vary across regions of the object.In one exemplary embodiment, fine ceramic powders or powder mixtureswith grain sizes typically less than 30 μm are used. The powder grainsize may vary from one region of the object to another, such as bypreparing a powder bed having at least two regions with at least twodifferent powder mixtures. The powder mixture may also comprise one ormore components that improve the densification/sintering by producing aliquid phase during the densification process (liquid-phase sintering).

A small residual porosity that may be present after laser sintering canbe densified further by additional sintering or hot isostatic pressing.The geometry of the ceramic shaped object may be stored in aComputer-Aided Design (CAD) data set. A computer may be configured tocontrol a laser system in response to the CAD data set for performinglaser sintering of the powder bed in such a way that the geometry of thecomponent is produced from a raw material powder bed.

The FIGURE shows a section through a ceramic mold shell 1 that isfabricated in accordance with aspects of the present invention. Theceramic mold shell 1 includes an outer envelope 4, which may be producedby means of a rapid prototyping process. The ceramic mold shell 1 may beused for a casting process, such as an investment casting process.Liquid metallic ceramic material may be cast into the cavity 10 formedby the outer envelope 4.

The ceramic mold shell 1 may also have a core 7 formed in its interiorso that a hollow shaped body can be produced during a casting process.The core 7 may be mechanically joined to the outer envelope 4 bysuitable affixing means, such as by a common base plate 6. The core 7may comprise a different material than the outer envelope 4, and theenvelope 4 may be layered or graded with different materials and/ordifferent densities in various regions. In the case of selective lasersintering, the laser beam generated by the laser may, for example, bemodulated to apply a greater or lower power to the regions in which thecore 7 is formed, so that the desired densification condition for thematerial of the core 7 is created. It is also contemplated for the laserpower to be locally varied, so that the outer envelope 4 has a highdensity in an inner region 13, where it is in contact with the moltenmaterial, and may have a lower density in an outer region 16. Othercombinations of materials, densities and regions may be achieved forvarious embodiments.

It is also possible for the topmost surface region 15 of the innerregion 13 to be formed as a porous layer that is, for example,approximately 1 mm thick while the remainder of region 13 is denselyformed. This embodiment of a ceramic mold shell 1 may be used forcasting a body that has a rough surface. Such a rough surface may beuseful, for example, if coatings are applied to the casting, so that thecoatings bond very successfully to the casting as a result of the roughsurface.

Aspects of the present invention may be particularly useful when theouter envelope 4 is produced together with the core 7 in a manner thatprovides differing materials and/or densities between the core 7 and theenvelope 4, such as by controlling the composition of the powder and theheat input provided by the laser in accordance with a desired controlstrategy. This considerably simplifies the production process whencompared to prior art methods. The ceramic mold shell 1 can be densifiedstill further by sintering or hot isostatic pressing after it has beenproduced by means of a rapid prototyping selective laser sinteringprocess. Consequently, ceramic rapid prototype models may be useddirectly for a casting process and do not form a pattern, but rather ashaped cast that is fully useable. Larger mold shells can also beassembled as an integrated unit during a single fabrication process.

In one embodiment, a powder bed is prepared having a first powder mix ina first region and a second powder mix in a second region. The chemicalcompositions and/or powder particles size distributions may be variedbetween the two regions, for example a first mixture in a first regionfor forming the envelope 4 and a second mixture in a second region forforming the core 7. The powders preferably comprise ceramics but maycomprise ceramic/alloy combinations. In one embodiment, silicon nitrideand yttrium powders are combined to provide a degree of liquid phasesintering. A laser may be controlled in accordance with a stored programset to apply heat to the two regions, with the amount of heat at anygiven location varying between the two regions, and possibly even withinone region, to achieve a desired degree of densification of the powders.The temperature necessary to achieve the desired densification in thefirst region may be different than the temperature necessary to achievethe desired densification in the second region, and the laser may becontrolled accordingly as it is scanned across the powder bed.Additional layers of powder and additional steps of laser heating may beadded to form a ceramic shape in accordance with the stored program. Theshape is formed as a net shape or a near net shape, with little or noadditional processing necessary before use. If desired, additionalheating or material infusion may be provided after the selective lasersintering process forms the shape.

While the preferred embodiments of the present invention have been shownand described herein, it will be obvious that such embodiments areprovided by way of example only. Numerous variations, changes andsubstitutions will occur to those of skill in the art without departingfrom the invention herein. Accordingly, it is intended that theinvention be limited only by the spirit and scope of the appendedclaims.

1-16. (canceled)
 17. A process for producing a ceramic shaped objectfrom ceramic powder, comprising: forming a first region of the shapedobject by a first laser sintering of a first ceramic powder; and forminga second region of the shaped object integral with the first region by asecond laser sintering of a second ceramic powder, wherein the formingof at least one of the first and second regions comprises controlling atleast one parameter selected to provide a different material property inthe first and second regions of the shaped object.
 18. The process ofclaim 17, wherein a ceramic mold is formed.
 19. The process of claim 17,wherein the material property is selected from the group consisting of:densification, porosity, surface roughness, and any combination thereof.20. The process of claim 17, further comprising controlling a laser beamgenerated during the first and second laser sintering processes toproduce a different sintering temperature over the first and secondregions of the object creating a different degree of densification inthe first and second regions of the shaped object.
 21. The process ofclaim 18, further comprising at least one of an additional lasersintering process and a hot isostatic pressing of the ceramic mold toachieve any further densification.
 22. The process of claim 17, furthercomprising accessing a computerized representation of the object andusing the computerized representation to control the process forproducing the ceramic shaped object.
 23. The process of claim 18,further controlling the process to form the first region of the ceramicmold to comprise a shell and the second region of the ceramic mold tocomprise a core disposed in a cavity of the shell.
 24. The process ofclaim 18, wherein the first region of the ceramic mold comprises aninner region and the second region of the ceramic mold comprises anouter region and the process is controlled so that the inner region isdenser than the outer region of the mold.
 25. The process of claim 17,further comprising using a ceramic powder or a powder mixture comprisinggrain sizes of less than 30 μm for at least one of the regions of theobject.
 26. The process of claim 17, further comprising using a ceramicpowder and a powder mixture comprising grain sizes of less than 30 μmfor at least one of the regions of the object.
 27. The process of claim17, wherein the ceramic powder comprises at least one ingredient thataffects densification and/or sintering of the ceramic powder byproducing a liquid phase for at least one of the regions of the object.28. The process of claim 18, wherein the process is controlled toprovide a surface in an inner region of the ceramic mold comprising asurface roughness different from an outer region of the ceramic mold.29. A mold for producing a ceramic shaped object, comprising: a firstregion of the ceramic shaped object formed by a first sintering of afirst ceramic powder; a second region of the ceramic shaped objectformed by a second sintering of a second ceramic powder and the secondregion is integral with the first region; and a parameter selected andcontrolled to provide a different material property in the first andsecond regions of the shaped object.
 30. The mold as claimed in claim29, wherein the mold is a ceramic mold.
 31. The mold as claimed in claim29, wherein the first ceramic powder comprises a first material and thesecond ceramic powder comprises a second material different than thefirst material.
 32. The mold as claimed in claim 29, wherein the firstregion comprises a first porosity and the second region comprises asecond porosity different than the first porosity.
 33. The mold asclaimed in claim 29, wherein the first region comprises a first densityand the second region comprises a second density different than thefirst density.
 34. The mold as claimed in claim 29, wherein the firstregion comprises a first surface roughness and the second regioncomprises a second surface roughness different than the first surfaceroughness.
 35. The mold as claimed in claim 29, wherein the firstceramic powder comprises a first average grain size and the secondceramic powder comprises a second average grain size.